In most conventional vehicular steering systems, an input device (e.g. steering wheel or hand-wheel) is connected to a steering actuator (e.g. steerable road wheel) via an articulated mechanical linkage. As such, any motion of the input device is translated to the steering actuator resulting in a motion of the steering actuator that directly corresponds to the motion of the input device.
However, with vehicular steering systems that employ active steering, such as in an automotive front-controlled steering system, a given motion of the hand-wheel may be supplemented by the motion of a differential steering device or actuator in a manner responsive to the yaw and/or lateral acceleration of the vehicle, resulting in a motion of the steerable road wheels that does not necessarily directly correspond to the motion of the hand-wheel. For example, at very low speeds as might be experienced in a parking situation, a relatively small rotation of the hand-wheel may be supplemented via active steering to provide an increased steering angle to the steerable road wheels.
Although several prototypes of differential steering devices have been developed, these are primarily planetary gear differentials having multiple planet planetary gears. These multiple planet planetary gears, which work on the difference principle of changing sense by reversing the direction of the rotation of input to output when the carrier is held fixed, have a number of shortcomings. First, multiple gear meshes are used to accommodate the differential function while maintaining sufficient torque capacity. Multiple planets have multiple paths of gear mesh and dimensional and positional variation of these paths may cause synchronization issues. As such, excess backlash (unproductive rotation) is introduced into these multiple planet designs in order to avoid tooth mesh interference in any one of the gear mesh paths. This is undesirable because backlash and friction are detrimental to steering feel and requires that the phasing of the gear teeth be tightly controlled, increasing the manufacturing and production costs.
Second, several parts are required to accommodate the differential function. This also increases system cost and containment space. Third, external sun gears meshing with external planet gears produce a low contact ratio and thus produces wear on the system. Lastly, the sense of the differential gears have to be corrected by additional means, such as adding an additional gear train to correct the direction. This increases system cost and containment space. This extra gear train also has the added disadvantage in that the center axis is not maintained through the device and thus the input to output ratio is at least 3:1. The problem with this large ratio is that road feedback is decreased which may adversely affect driver feel.